1. Field of the Invention
The invention relates to a material and a structural part, especially a fuel-rod sheathing tube, made from a zirconium alloy having the constituents of zircaloy, for a boiling-water nuclear reactor.
Due to its small neutron absorption cross section, zirconium which is obtained on a large scale as zirconium sponge and contains minor impurities of other elements, is used for fuel-assembly boxes, spacers, water pipes and other structural parts in the core of a nuclear reactor, especially for the sheathing tubes of fuel rods filled with oxidic nuclear fuel. In order to obtain sufficient chemical and mechanical stability in the coolant of the reactor core over the long service lives of the fuel assemblies, other elements, especially tin, iron, chromium and, if appropriate, nickel as well, are alloyed in small quantities to the zirconium sponge. Table 1 shows zirconium qualities and alloys which are standardized according to ASTM B 352 and 353 and which are cleared for use in nuclear reactors.
The development of those materials is based on corrosion tests under laboratory conditions, in which the chemical aggressiveness of the coolant is increased by means of a considerable rise in temperature, in order to perform tests within short test periods of the corrosion resistance of the material which is exposed to the corrosive coolant over a long service life, during reactor operation under actual conditions. As a result, the weight increase caused by the formation of zirconium oxide is normally used as a measure of the corrosion. It is shown therefrom that, in the first place, a surface layer of zirconium oxide forms quickly, and the aqueous coolant has to diffuse through the surface layer before it can cause corrosion in layers of the material which are located further inwards. The alloyed constituents reduce the oxide formation on the surface and the diffusion of the coolant and thus increase the corrosion resistance of the workpiece.
A particular feature of the boiling-water reactor is that the workpieces have to be corrosion-resistant both to liquid water and to water vapor. According to experience, measures which are effective for water vapor do not always also have an effect in the case of liquid water, and therefore, in particular, laboratory tests at high temperatures and pressures, in which a distinction between the liquid and the vapor phase can no longer be made, do not have any evidential force with regard to corrosion under the operating conditions of boiling-water reactors.
Furthermore, foreign substances, which are contained in the water and which, for example, originate from leaks in condensers or from other components of the coolant circuit or are added for the protection of those components, can concentrate in the non-evaporated liquid droplets and thereby aggravate the corrosion conditions as soon as vapor formation occurs on the hot fuel rods.
In reactor operation under actual conditions, at least in boiling-water reactors, in addition to the long-term corrosion which is distributed relatively uniformly over the surface, there often occurs a local corrosion which emanates from specific corrosion sources, for example regions of differing composition, and which can lead to progressive local destructions of the workpiece. This "nodular" corrosion appears primarily under the operating conditions of the boiling-water reactor, that is to say under the radiation load on the material and the special chemical and thermal influences which cannot be simulated well by the corrosion tests under laboratory conditions.
The hydrogen released during such oxidation is important for the mechanical stability of the workpiece, since it diffuses into the workpiece according to the locally forming hydrogen partial pressure and leads there to a specific hydrogen concentration and hydrogenation. However, hydrogenated regions of zirconium exhibit pronounced embrittlement, so that the workpiece can no longer withstand mechanical loads. The diffusion and absorption of hydrogen and the formation of hydrogenated regions are therefore also to be reduced by the alloyed constituents, for not only external influences occurring during reactor operation, such as, for example, vibrations caused by the flowing coolant, but also thermal effects, an increase in volume occurring during the oxidation and hydrogenation of the material and other operation-related variations lead to unavoidable mechanical stresses. At the same time, it is particularly necessary to also bear in mind that the neutron radiation can lead to variations in the material structure and in the dimensions of the workpieces (in regard to fuel rods, for example, to an increasing length and a decreasing diameter of the sheathing tube). Since those loads arise only in reactor operation under actual conditions, it is necessary from time to time to carry out a thorough inspection of the workpieces used in the reactor, in order to estimate their further behavior, and if appropriate to carry out an exchange and fix the material strengths which are necessary for the workpieces.
Published European Application No. 0 085 553 A2 describes a sheathing tube of a fuel rod which is formed of zircaloy and which, as a result of beta annealing and subsequent quenching, acquires a structure with finely distributed secondary precipitations that are maintained during subsequent fine-forming operations at temperatures of below 620.degree. C. and which have a mean particle size of approximately 0.05 to 0.07 .mu.m. Zircaloy which used in that case is formed of 1.49% Sn, 0.21% Fe, 0.11% Cr, 0.0055% Ni, 0.0082% Si, 0.113% O, 0.0147% C, and the remainder being zirconium with impurities, wherein all particulars are given in % by weight. In an autoclave under vapor at 454.degree. C., the sheathing tube shows less corrosion than conventional zircaloy 4 which is hot-formed at 780.degree. C.
Published UK Application GB 2 172 737 A describes a sheathing tube made from zircaloy 2 (1.45% Sn, 0.16% Fe, 0.12% Cr, 0.05% Ni) with an inner lining made from unalloyed zirconium, the iron content of which is between 0.025 and 0.100%. In another sheathing tube (known from Published European Application No. 0 194 797 A1), an outer layer of zircaloy 2 or zircaloy carries an inner lining made from zirconium with 0.4 to 0.6% Sn, 0.5 to 1.4% Fe and 0.01 to 0.07% O.